This invention relates generally to the production of improved strains of forest trees and more particularly to a method for inducing juvenile flowering in certain gymnosperms, especially those within the family Pinaceae.
For many years foward-looking land managers within the forest products industry have been reforesting cut-over land in order to produce a new crop of timber or pulp wood trees. In the early days of the program, it was customary to leave individual mature trees, or a block of trees, uncut to provide a seed source for the following generation. While generally successful, this method has a number of serious limitations. Many desirable species of trees produce good seed crops only at infrequent and undependable intervals. Douglas-fir, Pseudotsuga menziesii, and loblolly pine, Pinus taeda are two such examples. Good cone crops typically occur every five to seven years. In this period of time it is not uncommon for competing brush or less desirable tree species to take over the land. The result of depending entirely on natural reseeding was often to have large areas of land which were poorly stocked in the preferred species.
One adverse effect of depending on natural seeding was often recognized and unintended. Quite often the remaining seed trees were chosen because they were of a form or condition that caused them to be of lower economic value than the surrounding trees in the stand. No thought was given to the possibility that the poor form might be a heritable genetic characteristic which would be transmitted to a predominant number of trees in the succeeding forest. Rarely was there any genetic gain from stand to stand. Much more often it was neutral and very frequently is was actually dysgenic for the reasons just given.
Even in the first quarter of the present century, a few progressive foresters maintained small nurseries to supply seedlings for reforestation. Unfortunately, at that time the effort was not widespread because of adverse economic factors and because so few people had vision to realize that the great stands of virgin timber in the world's forests would not last forever. It soon became painfully apparent that this was not the case. About the same time, forest managers began to see the limitations of natural restocking. Late in the second quarter of the century nurseries began to supply millions of tree seedlings which were hand planted to restock cut-over forest land. Other techniques, such as aerial reseeding, were also brought into play to minimize the time lag between harvesting one forest crop and beginning to grow another one.
For much of the time that tree nurseries have been in existence, they have depended on wild seed for production of seedlings. Collection of the cones of various evergreens became an important source of income for many rural people. In good seed years, more than adequate seed was collected for the next year's needs. The balance would be put into cold storage for subsequent years, thus minimizing somewhat the effect of the cyclic nature of seed production.
Wild seed is drawn from an enormously varied gene pool. It was not long before foresters began to recognize that some seedlings grew far better in localized enviornments than others. In the Douglas-fir region, for example, it was found to be important to plant seedlings at the same approximate altitude from which the seed had been obtained. Soon it was realized that many other tree characteristics were heritable. While these traits varied from species to species, among them might be mentioned growth rates, the tendency to have straight or crooked stems, wood density, light vs. heavy limbs, etc. Nursery managers thus began searching their forest for wild trees that would excel in one or more desirable characteristics. The principle trait sought was rapid growth rate. By restocking with trees having faster growth, years could be trimmed from the crop rotation cycle. Cuttings were taken from superior trees and grafted to hardy rootstocks. These grafted trees, in turn, were set out to become the first commercial seed orchards. This marked the beginning of overcoming the dependence on the vagaries of nature by collection of wild seed. It also marked the beginning of the genetic improvement of forest crops, much as man has done with other agricultural crops for several thousands of years.
Trees, however, represent a far more difficult problem to the breeder than do annual crops such as corn or wheat. Depending on the species, it may take from 15 to 50 years for a new generation to produce seeds of its own. However, several generations of breeding are required to maximize genetic improvement. This produces a very serious handicap to the tree breeder. He must take cross pollinated seed from his first generation orchard and plant a certain amount of it in genetic test plots. The best trees from the F.sub.1 generation are then selected and grafted for additional cross breeding experiments. Some of the seed from these matings must again be set out into genetic test plots and the best trees once more selected, with the cycle repeating itself about every 20 years using currently available methods.
Since the object of a forest tree breeding program is to be able to supply genetically superior seedlings by the hundreds of millions in the shortest time possible, it is critical to reduce the generation time of stock in the breeding program. Unless major reductions could be made, it was apparent that for many species the tree scientists working on improvement programs would not be alive to realize the fruits of their labors. The maximum benefits would also be denied to the public with an ever increasing need for wood fiber.
Approximately thirty species of gymnosperms, the so called softwoods, comprise the great bulk of the commercially important timber species useful for construction lumber. Among these are the pines which include loblolly pine (Pinus taeda), slash pine (Pinus elliotii), longleaf pine (Pinus palustris), shortleaf pine (Pinus echinata), ponderosa pine (Pinus ponderosa), red pine (Pinus resinosa), jack pine (Pinus banksiana), Eastern white pine (Pinus strobus), Western white pine (Pinus monticola), sugar pine (Pinus lambertinana), lodgepole pine (Pinus contorta); Douglas-fir (Pseudotsuga mensiesii); Western hemlock (Tsuga canadensis); Sitka spruce (Picea glauca); redwood (Sequoia sempervirens); the true firs including silver fir (Abies amabilis), grand fir (Abies grandis), noble fir (Abies procera), white fir (Abies concolor), balsam fir (Abies balsamea); and the cedars which include Western red cedar (Thuja plicata), incense cedar (Libocedrus decurrens), Port Orford cedar (Chamaecyparis lawsoniana), and Alaska yellow-cedar (Chamaecyparis nootkatensis), and Western larch (Laryx occidentalis).
This list is not intended to be all-inclusive of the commercially important softwood species. It does, however, include the major ones which are commercially harvested and are becoming subject to intensive silvicultural management. Several species might be singles out as being particularly important. These are the four so-called southern yellow pines, slash, longleaf, shortleaf, and loblolly; ponderosa pine; Western hemlock; and Douglas-fir. Of this last group, loblolly pine and Douglas-fir have been the subject of intensive tree improvement breeding programs.
The problem of inducing early flowering in tree breeding programs is not a new one nor is it limited to forest species. It is one that has been of concern to orchardists for at least a century and a half. T. A. Knight, in a paper published in 1820, observed that certain fruit trees can be made to flower at earlier ages by stem girdling (Trans. Hort. Soc. London 4:159-162). This procedure is used to the present day and is effective on certain gymnosperms as well as dicotyledon angiosperms.
R. M. Sachs, who cites Knight in a paper published in 1977, contemplates the biochemical mechanism of flowering induction (Hort. Sci. 12:220-222). He generally agrees with Knight and suggests that "a nutrient diversion hypothesis . . . may best account for promotion of flowering" by treatments that induce stress in the trees. By this he is referring to means that appear to switch nutrients away from promotion of vegetative growth to inducement of flowering.
J. A. Romberger and R. A. Gregory (In Proc. 3rd North Amer. For. Biol. Workshop, Colorado State Univ., Fort Collins, Colo., pp. 132-147. 1974.) believe that in woody trees there are a number of interrelated systems, all of which must be permissive if flowering is to occur. They ponder the conditions which cause apparently similar cell clusters to differentiate in one of several directions, e.g., male or female strobili or vegetative buds.
D. I. Jackson and G. B. Sweet, in what is basically a massive literature review published 1971, draw some conclusions on bud differentiation from a somewhat different viewpoint. (Hort. Abstr. 42:9-24). They note that in Pinus species primordium formation does not occur while extension growth is is active. In young trees, flowering is very infrequent and the trees are considered juvenile. They feel further that the route from "meristematic initials" to flowering buds is a long one that is "easily reversible in its early stages." Again, in Pinus, they state that either flowers, needle fascicles or branches may form in the axils of the scale leaves at nodes. They suggest that the first determination that takes place is between vegetative or floral growth. In the latter case sexual determination is a step which occurs even further downstream. These authorities suggest the term "phase change" for the transition time when trees change from an unfruiting juvenile to a fruiting adult or mature stage.
R. H. Zimmerman published a similar survey in 1973, although it was heavily directed to the problems of juvenility and flowering of fruit trees (Hort. Sci. 7(5):447-455). He did, however, deal somewhat with Pinus and concluded that juvenility was related to a number of factors which included photoperiod, temperature, water supply and nutrients. He noted that top working; i.e., grafting of a juvenile scion into a mature blossoming tree, was one means of inducing earlier male blossoms in pine. Zimmerman made a further statement which can only be bound to strike dismay into the hearts of tree breaders. This was his observation in regard to early flowering that different plant species respond to different treatments and there is little or no predictability from one species to another.
A paper published in 1977 by the present inventor (Greenwood, M. S., In Proc. 3rd World Consultation on Forest Tree Breeding, Canberra, Australia, CSRIO. FO-FTB--4/25) looked specifically at the problem of producing early flowering in loblolly pine (Pinus taeda). A number of treatments were found to be effective in induction of female flowering, e.g., pot culture, gibberellins, water stress, girdling, or some combination of these. None of these treatments were effective in inducing male flowering, however. Zimmerman's statement of unpredictability is well supported in this work. Ethrel (2-chloroethyl phosphonic acid) treatment is effective in inducing both male and female flowering in some species. The same was true for the gibberellin GA 4/7 which is known to increase both male and female flowering in Douglas-fir. Neither of these treatments were effective in promoting male flowering by Pinus taeda.
Accordingly, one object with the present invention is to provide an improved method of inducing juvenile male and female flowering in gymnosperms of the family Pinaceae.
A further object is to provide an effective method for inducing juvenile male flowering in species of the genus Pinus.